From Agronomists of the
Potash & Phosphate Institute
655 Engineering Drive, Suite 110
Norcross, Georgia 30092-2837
Phone (770) 447-0335

Winter 1997, No. 6


        Seeing is believing...right! Not always, not when it comes to soil variability. The variability in some fields is readily apparent, while in others it's not. But that doesn't mean it's not present. Even "uniform" fields may be more variable than you think. Soil scientists have known that for years, but the advent of yield monitors is making believers out of a lot more people.

    Soil properties change in all directions across the landscape. Variables include topography, moisture content, drainage, soil texture, topsoil depth, soil structure, compaction, organic matter content, soil pH, salinity and fertility...the list can go on and on. These properties can vary greatly over short distances, resulting in differences in crop yield and quality, and erratic performances of herbicides and fertilizers.

   Understanding the cause of variation helps you know how to measure it and manage it. Soil variability is naturally caused by climate, topography, parent materials, vegetation, time, and man. Human influence_how we've farmed...can cause variability and how we farm can correct it.

    Variation due to topography is easy to see. Eroded knolls stand out, but terrain also impacts the distribution of moisture and nutrients. Mobile nutrients like nitrate, sulfate or chloride often accumulate in the lower slope positions because of water movement downhill. Immobile nutrients like phosphorus or potassium are tightly bound to soil particles, so only move if the soil moves due to erosion or tillage. With higher fertility and more moisture, the lower slopes often produce more crop, which extracts more nutrients, resulting in more variability. Variation on flat land is not obvious but can be just as great. Aside from underlying soil strata or rocks, disease and pest pressure cause local variations in crop growth. Reduced crop growth means less nutrient and water use in some spots. Weed pressures may increase water and nutrient removal in other spots. Past farming practices, such as straw and chaff distribution at harvest, can cause localized differences in surface residues that can impact soil moisture, surface temperatures and nutrient tie-up.

    Variability is present and we can measure it. More intensive soil and tissue sampling, black and white aerial photography, infrared photography, satellite imagery, yield monitors, protein monitors, and other techniques are part of the "tool box" we should be using.

    Awareness is the first step in recognizing and correcting variability. Site-specific management, or precision farming, stems from awareness of variability. If we are aware of variability, we can do something about it. We can modify our farming practices and take advantage of new tools. For example, variable rate application equipment makes it possible to differentially apply fertilizers and herbicides and adjust plant populations.

    Level out the playing field by recognizing soil variability and taking steps to correct it.

--- TLR ---

For more information, contact Dr. Terry L. Roberts, Western Canada Director, PPI, Suite 704, CN Tower, Midtown Plaza, Saskatoon, Saskatchewan, Canada S7K 1J5. Phone (306) 652-3535. E-mail:
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